> ## Content Index > Fetch the complete content index at: https://research-pop.com/llms.txt > Use this file to discover other available public pages before exploring further. # Ulm University Team Uses Raman Overtones to Reassign the Nickel-Anode Surface during Alkaline Electrolysis - URL: https://research-pop.com/ulm-university-team-uses-raman-overtones-to-reassign-the-nickel-anode-surface-during-alkaline-electrolysis/ - Published: 2026-09-14T12:18:26.000Z - Updated: 2026-09-14T12:18:26.000Z - Author: ResearchPOP - Tags: Chemistry ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/Screenshot-2026-09-11-at-22.49.52.png) **Source**: [https://www.nature.com/articles/s41929-026-01613-9](https://www.nature.com/articles/s41929-026-01613-9?ref=research-pop.com) ## At a glance Nickel-based anodes are active and relatively low-cost catalysts for the oxygen evolution reaction (OER) in alkaline water electrolysis. Under operating conditions, their oxidized surfaces have commonly been described as nickel oxyhydroxide (NiOOH). A team from Ulm University has now combined isotope-labelled in situ surface-enhanced Raman spectroscopy (SERS) with density functional theory (DFT) calculations to examine the structure of this surface and the origin of its Raman signals. The results assign the catalytically active surface layer to hydrogen-free nickel dioxide (NiO₂) rather than NiOOH. The researchers also attribute broad Raman bands previously associated with surface superoxide species to overtones and combination bands derived from the fundamental lattice vibrations of NiO₂. Measurements under different potentials and electrolyte conditions further connect shifts in these signals to a vibrational Stark effect. Together, the findings provide a revised structural picture of nickel anodes during alkaline oxygen evolution. ## Background Raman spectroscopy identifies materials through their vibrational signatures and can probe catalysts directly in aqueous environments under operating conditions. Surface-enhanced Raman spectroscopy strengthens weak signals through plasmonic enhancement, making it particularly useful for studying electrode surfaces. Most Raman analyses focus on fundamental vibrational modes. In real materials, anharmonic effects can also produce overtones at approximately integer multiples of fundamental frequencies and combination bands formed from sums of different modes. Recognizing these higher-order signals is important because their assignment affects how the structure and chemistry of a working catalyst are interpreted. Nickel (oxy)hydroxides are widely studied as OER catalysts in alkaline electrolysers. When Ni(OH)₂ is oxidized at positive potentials, the resulting material is generally described as NiOOH, including the β- and γ-NiOOH phases. Raman spectra recorded under these conditions typically contain two strong bands near 479 and 560 cm⁻¹, alongside broader features between approximately 900 and 1,200 cm⁻¹. The higher-wavenumber region has often been associated with peroxide or superoxide species on the surface. In this study, the researchers investigated which nickel oxide structure accounts for the Raman spectrum observed near and beyond OER onset. They also examined whether the high-wavenumber features arise from an additional oxygen species or from higher-order vibrations of the nickel oxide lattice. ## Research questions and approach The team deposited nickel films on roughened gold wires and recorded in situ surface-enhanced Raman spectra in alkaline electrolytes while varying the applied potential, cation, pH, total ion concentration and isotopic composition. Measurements in ordinary water were compared with experiments using deuterated and oxygen-18-labelled electrolytes to track the involvement of hydrogen and oxygen in the observed vibrations. The researchers separately modelled bulk and surface structures of NiOOH and hydrogen-free NiO₂ using periodic DFT. They calculated the Raman-active vibrational modes of each structure and compared the predicted peak positions and relative intensities with the experimental spectra. Reported Ni–Ni and Ni–O interatomic distances were also considered in the structural analysis. To examine the broad high-wavenumber bands, the team compared their positions with multiples and sums of the experimentally measured fundamental frequencies. Additional potential-dependent SERS measurements on cobalt- and manganese-based oxides were used to explore how these higher-order signals relate to material structure. ## Inside the study Before oxidation, the Ni(OH)₂ surface displayed three Raman features at approximately 326, 456 and 501 cm⁻¹. As the potential increased to around the onset of oxygen evolution, these features disappeared and were replaced by two intense modes at about 479 and 560 cm⁻¹, together with a shoulder near 610 cm⁻¹. Broad bands also appeared in the regions of approximately 900–1,200 cm⁻¹ and 1,400–1,800 cm⁻¹. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-11.png) **Fig. 1: Cyclic voltammogram and surface-enhanced Raman spectra of a Ni film supported on a Au wire.** Changing the electrolyte cation among Li⁺, Na⁺, K⁺ and Cs⁺ at a concentration of 0.01 M produced nearly identical Raman features. When the researchers changed the pH while maintaining a similar total ion concentration, the principal peak positions also remained essentially unchanged. These measurements showed that the main spectral features were retained across the tested electrolyte compositions. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-12.png) **Fig. 2: Surface-enhanced Raman spectra of oxidized Ni films on a Au wire in different electrolytes.** Isotope-labelled experiments provided further information about the surface structure. Replacing ordinary water with D₂O produced no measurable shift in the principal nickel oxide features, apart from the expected change in the water bending vibration. The authors interpreted this lack of a deuterium response as evidence that the SERS-probed oxidized surface does not contain hydrogen. In contrast, oxygen-18 labelling shifted the fundamental nickel–oxygen modes from approximately 479, 560 and 610 cm⁻¹ to lower wavenumbers of about 454, 529 and 590 cm⁻¹. The broad higher-wavenumber bands shifted at the same time, demonstrating that these signals are linked to vibrations involving oxygen atoms in the nickel oxide structure. The DFT calculations compared bulk and surface models of both NiOOH and NiO₂. The calculated NiOOH spectra contained several modes below 600 cm⁻¹ and predicted OH-related vibrations that did not reproduce the experimental pattern. By comparison, the NiO₂ models consistently produced two Raman-active modes close to the dominant experimental peaks at 479 and 560 cm⁻¹. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-13.png) **Fig. 3: Density functional theory-computed structures and Raman spectra compared with experimental data.** The calculated surface model of NiO₂ also reproduced the relative intensities of the two principal peaks more closely than the bulk model. The calculations identified additional nominally Raman-inactive modes near the weaker experimental features. The authors proposed that surface distortions and the interfacial electric field can relax the selection rules and allow these modes to appear in the measured spectrum. The structural comparison provided further support for the NiO₂ assignment. Calculated Ni–Ni and Ni–O distances for NiO₂ agreed with values reported from extended X-ray absorption fine structure measurements and previous computational studies. Raman spectra previously reported for Li₀.₀₁NiO₂ containing Ni(IV) species also closely resembled the spectra recorded in this work. Based on the combined isotope experiments, calculated Raman spectra and structural data, the authors assigned the SERS-sensitive terminating layer under OER conditions to NiO₂. The team next analysed the broad region between 900 and 1,200 cm⁻¹. Using the fundamental frequencies measured in each experiment, they positioned components corresponding to twice each fundamental frequency and to the sum of the two frequencies. Together, these overtone and combination components reproduced the broad experimental envelope. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-14.png) **Fig. 4: Overtone deconvolution of surface-enhanced Raman spectra of oxidized Ni in pure and labelled water.** The same approach accounted for higher-order bands in the 1,400–1,800 cm⁻¹ region. When oxygen-18 labelling shifted the fundamental modes, the higher-wavenumber components moved according to the corresponding multiples and sums. The researchers therefore assigned these broad regions to overtones and combination bands of the NiO₂ lattice vibrations. To explore the occurrence of similar signals in other materials, the team performed potential-dependent SERS measurements on cobalt- and manganese-based oxides. Layered CoOOH showed an overtone region associated with its fundamental modes, whereas the more rigid spinel Co₃O₄ phase did not show the same pronounced pattern. At higher potential, manganese oxide spectra associated with layered MnO₂ phases also displayed higher-order features that followed the fundamental modes. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-15.png) **Fig. 5: Surface-enhanced Raman spectra of a Au wire supported Co- and Mn-based oxides at different potentials.** From these comparisons, the authors suggested that pronounced overtone signals occur predominantly in layered materials. Weak interlayer interactions, defects and mismatches between layers may increase anharmonicity and enhance the higher-order vibrations. Finally, the researchers examined how the nickel oxide Raman spectrum changed with electrochemical conditions. Increasing the applied potential shifted the fundamental peaks, changed their relative intensities and reshaped the overtone region. Related changes were observed when the KOH concentration was varied. By contrast, changing the pH while keeping the total ion concentration similar produced little change in the peak positions. ![](https://storage.ghost.io/c/7c/5e/7c5e0911-739b-4a35-94eb-3e810bf78ba7/content/images/2026/09/image-16.png) **Fig. 6: Vibrational Stark effect in surface-enhanced Raman spectra of NiO2.** The authors attributed these potential- and concentration-dependent spectral changes to a vibrational Stark effect. In this interpretation, changes in the applied potential and the accumulation of ions at the electrode interface modify the local electric field. This field alters the vibrational frequencies and Raman intensities of the surface NiO₂ layer without requiring a change in its chemical identity. ## Takeaways and outlook By combining isotope-labelled in situ SERS with DFT-calculated Raman spectra, the study assigns the oxidized surface of nickel anodes under OER conditions to a terminating layer of Ni(IV)O₂. It further identifies the broad bands at higher wavenumbers as overtones and combination bands derived from the fundamental NiO₂ lattice modes and connects their response to electrochemical conditions with a vibrational Stark effect. This structural assignment changes the surface model used to describe alkaline oxygen evolution on nickel anodes. The authors discuss possible adsorbate-evolution and lattice-oxygen pathways on NiO₂ and highlight the importance of considering the hydrogen-free terminating layer when developing mechanistic descriptions. The SERS measurements emphasize the near-surface region, estimated under the conditions of this study to extend less than 3.2 nm, or fewer than seven monolayers. The authors therefore distinguish the NiO₂ surface termination identified here from the composition of the underlying bulk material. Future surface-sensitive operando measurements, together with studies of NiO₂ edges, defects and grain boundaries, could further connect this structure with the elementary steps of oxygen evolution. ## About the researchers Justus Leist (Ulm University) is the first author. Annika Neufischer is affiliated with Ulm University. Timo Jacob (Ulm University, Helmholtz Institute Ulm and Karlsruhe Institute of Technology) and Albert K. Engstfeld (Ulm University) are the corresponding authors. ## Original research Justus Leist, Annika Neufischer, Timo Jacob and Albert K. Engstfeld. “Consequences of overtones in Raman spectra for assigning nickel anode surface structures as NiO₂ during alkaline electrolysis.” Nature Catalysis (2026). DOI: [https://doi.org/10.1038/s41929-026-01613-9](https://doi.org/10.1038/s41929-026-01613-9?ref=research-pop.com). Published online: 11 September 2026 Open access under CC BY-NC-ND 4.0\. 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